The present invention relates to an arrangement and a method for determining a vehicle speed behaviour model for a leading vehicle. The invention further relates to a computer program adapted to perform such a method when said program is run on a computer, and a computer program product comprising such program code means stored on a computer readable medium. The arrangement is especially suitable for vehicles having an adaptive cruise control system.
Modern vehicles are often provided with a cruise control system in order to improve the comfort of a driver. Some cruise control systems are provided with further functions, such as an adaptive control system that uses distance measuring equipment to monitor the distance to a vehicle in front of the own vehicle, in order to be able to adapt the cruise speed to the speed of the vehicle in front. Such a system will improve the safety of the cruise control system since a minimum distance to the vehicle in front will be maintained. Some systems will also brake the vehicle if the minimum distance is not maintained, or if the distance to the vehicle in front decreases at a certain rate. The cruise control system may be adapted to maintain a set speed by using only the accelerator, or by also using the brakes of the vehicle. This is especially common in heavy vehicles which are equipped with auxiliary brakes, such as engine brakes and retarders.
For a vehicle travelling behind another vehicle or a group of vehicles, a good time gap (or distance) control is essential in order to travel comfortably, safe and efficient and at the same time balancing the desire to reach the set speed of the cruise control with the need to keep a safe time gap to the vehicle in front. When travelling with an adaptive cruise control engaged, which uses a forward looking sensor, the set speed of the host vehicle is changed when the distance to the vehicle in front changes. With more advanced cruise control systems, often referred to as a dynamic cruise control system (e.g. using map data and sensors), the set speed of the host vehicle may be changed for example in curves, when new speed limits are found, over a crest and in the end of a downhill in order to save fuel and increase safety. To be able to use such fuel saving features of the cruise control, the time gap control must take the distance to the leading vehicle into account. When the distance to the leading vehicle changes, e.g. due to a changed speed of the leading vehicle, the time gap control might not be able to keep a safe distance to the leading vehicle, or the cruise control may not be able to hold the set vehicle speed.
If an automatic speed control like a dynamic cruise control performs badly due to above mentioned reasons, fuel efficiency, safety or comfort will be suffering and the driver may turn the system off.
The cruise control system has a regulating range in which the actual speed value is maintained. For a system using only the accelerator to maintain the speed, an excessive speed of a few percent may be allowed. Typically, the speed may vary between e.g. 68 to 72 km/h when the speed is set to 70 km/h. It may be possible to set the maximal allowed excessive speed value for the vehicle. In this way, the system can allow the vehicle to travel at an excessive speed at certain occasions, e.g. when travelling on a slight descent.
When the brake system is also used to maintain the speed of the vehicle, referred to as a brake cruise system, a specified excessive speed can also be allowed for a vehicle rolling down a fairly steep downhill slope. An allowed excessive speed may help to preserve energy by using the momentum of the vehicle. The limit for the allowed excessive speed must of course be set to a value that does not endanger other vehicles.
For a vehicle having, an adaptive cruise control, a distance measuring device such as a radar measures the distance to a vehicle ahead, and regulates the set speed of the cruise control such that a predefined distance or time gap is preserved between the own vehicle and the vehicle ahead. When the vehicle ahead changes its speed, the adaptive cruise control will preserve the predefined distance or time gap within the selected cruise control range. If the vehicle ahead brakes rapidly, a warning message and/or an automatic brake function will be activated in the own vehicle. Such a system works fine for keeping a predefined distance to a vehicle ahead and for avoiding an accident.
There are however situations in which an adaptive cruise control does not control the speed of the vehicle in a fuel efficient way. Such a situation may e.g. be when the vehicle ahead is not controlled in the same way as the own vehicle. If the vehicle ahead e.g. reaches an ascent and loses speed rapidly, the adaptive cruise control of the on vehicle will apply the brakes such that the distance to the vehicle in front is retained. In this way, the own vehicle will lose momentum which affects the fuel, efficiency negatively. Since the cruise control system cannot take account for this rapid speed loss, it cannot adapt the speed control for such a situation. Another situation may be when the cruise control of the vehicle ahead is using a greater speed regulating range than the own vehicle. Also in this situation, the cruise control will not be able to adapt to the speed of the vehicle ahead in a fuel efficient way.
There is thus a need for a system that can adapt to a vehicle ahead in a fuel efficient way.
It is desirable to provide an arrangement for determining a vehicle speed behaviour model for a leading vehicle. It is also desirable to provide a method for determining a vehicle speed behaviour model for a leading vehicle.
In an arrangement for determining a vehicle speed behaviour model for a leading vehicle according, to an aspect of the invention, where the leading vehicle drives along a path in front of a trailing host vehicle and where the determination is done by a control unit in the host vehicle, where the host vehicle comprises means for measuring the time gap to the leading vehicle, means for measuring the speed of the host vehicle, means for obtaining external information regarding an upcoming change in a road condition, the problem is solved in that the control unit is adapted for monitoring the change in vehicle speed of the leading vehicle, and for determining a vehicle speed behaviour model for the leading vehicle based on the monitored change in vehicle speed of the leading vehicle.
By this first embodiment of the arrangement, the control unit tracks the time gap to the leading vehicle, preferably by measuring the time gap with a predefined sample rate. The control unit further tracks the speed of the host vehicle, preferably with a predefined sample rate. The control unit will receive external information regarding an upcoming change in a road condition, e.g. from a map database or from a road sign having, a radio transmitter. The control unit will monitor the change in vehicle speed of the leading vehicle in dependency of the change in road condition, and will from this speed change determine a vehicle speed behaviour model for the leading vehicle. The vehicle speed behaviour model may be adapted to the type of road change condition. The vehicle speed behaviour of the leading vehicle may be different for an ascent than for a change in allowed road speed. The vehicle speed behaviour may further be dependent on the driver of the leading vehicle, or may be dependent on the type of cruise control that the leading vehicle is provided with. The speed behaviour will e.g. differ between a conventional cruise control and an adaptive cruise control.
The vehicle speed behaviour model can be used by the host vehicle to predict the behaviour of the leading vehicle. Such a behaviour may e.g. be how the leading vehicle reacts on a change in road condition. When a change in the allowed road speed occurs, the leading vehicle may either roll until the new, lower allowed speed is obtained, or it may brake in order to reach the lower speed faster. By knowing this behaviour in advance, the cruise control of the host vehicle can adapt to the behaviour e.g. by temporarily leaving a longer distance to the leading vehicle at a change in road condition, such that an unnecessary braking of the vehicle is avoided.
The vehicle speed behaviour model can also be used when the host vehicle is driven without cruise control, e.g. by adapting a forward warning system used to give a warning when the leading vehicle is within a predefined distance. If it is known that the leading vehicle brakes hard in order to reduce the speed, the predefined distance for the warning can temporarily be increased somewhat. The vehicle speed behaviour model can e.g. also be used by an anticipative driveline which optimises gear changes automatically, in eco-roll systems and in hybrid systems in order to blend power sources.
In an advantageous development of the inventive arrangement, the arrangement comprises a forward looking sensor that can monitor also a vehicle ahead of the leading vehicle. In this way, the vehicle speed behaviour model will also take account of how the leading vehicle behaves with regard to the vehicle ahead of the leading vehicle. The comfort of the driver in the host vehicle may thus be further improved, together with an improvement of the fuel efficiency.
In a method for determining a vehicle speed behaviour model for a leading vehicle, where the leading vehicle drives along a path in front of a trailing host vehicle and where the determination is done in the host vehicle, the steps of tracking the time gap to the leading vehicle, tracking the speed of the host vehicle, obtaining external information regarding an upcoming change in a road condition, which may alter the speed of the host vehicle, monitoring the change in vehicle speed of the leading vehicle, and determining a vehicle speed behaviour model for the leading vehicle based on the monitored changes in vehicle speed of the leading vehicle are comprised.
With the inventive method, an improved behaviour of the cruise control system of a vehicle can be obtained, in which the cruise control system will adapt to the speed behaviour of the vehicle ahead. In this way, the fuel consumption can be lowered compared with an adaptive cruise control regulating on the distance or the time gap to the vehicle ahead.